Many electronic and electrical devices, for example, recording devices such as personal video recorders (PVRs), set top boxes (STBs), personal computers (PCs), and the like, rely on undisturbed alternating current (AC) derived power to complete critical system operations. AC power sources can suffer from a variety of long-term and transient disturbances. The term “power disturbance”, as used herein, refers to any out of tolerance condition of AC power input to any electrical, electromechanical or electronic device. Out of tolerance conditions include out of regulation conditions. Examples of power disturbances include variations in the frequency and/or amplitude of the input power. Though the former is usually acceptably stable, amplitude variations occur frequently. Unfortunately, in some cases AC power is lost altogether.
Computer systems, set top boxes, PVRs, DVD and CD recorders, etc. typically employ various memory devices to store program and system information. Memory devices include hard disk drives, recordable disks, semiconductor memory devices and the like. A power failure, brownout, or other disturbance can occur at any time. When power is disturbed in the middle of a memory write operation, data in the memory is sometimes corrupted. In some cases, a device will fail to operate after a power disturbance. For example a computer will sometimes fail to boot up because critical system information stored in memory was corrupted when a power disturbance interrupted a write operation. In addition, many electro-mechanical devices rely on undisturbed power to complete critical mechanical movements. Therefore, vulnerability to power disturbances remains a problem for many electronic, electrical and electro-mechanical devices.
Some conventional systems attempt to solve the problem by means of an early power fail (EPF) warning circuit to indicate an impending power disturbance. Existing EPF warning approaches typically rely upon voltage monitoring of a power supply output to detect disturbances. This approach has drawbacks. A switch-mode power supply, for example, may stay in regulation for some time after the input AC voltage drops, due to the charge held on its input capacitor. Therefore, any warning based on a decrease in output voltage level is delayed in time from the actual occurrence of the corresponding power disturbance. This delay time is typically a function of the discharge time constant of the input capacitor. Because of this delay time, prior art approaches often fail to provide adequate advance warning of an AC power disturbance.
Accordingly, circuits and methods are needed that are capable of indicating a power disturbance with decreased delay.